The present invention relates to imaging sheets which can be used in displays that selectively transmit or reflect light.
Currently, information is displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically writable areas to carry ticketing or financial information, however magnetically written data is not visible.
A structure is disclosed in PCT/WO 97/04398, entitled xe2x80x9cElectronic Book With Multiple Display Pagesxe2x80x9d which is a thorough recitation of the art of thin, electronically written display technologies. Disclosed is the assembling of multiple display sheets that are bound into a xe2x80x9cbookxe2x80x9d, each sheet is arranged to be individually addressed. The patent recites prior art in forming thin, electronically written pages, including flexible sheets, image modulating material formed from a bi-stable liquid crystal system, and thin metallic conductor lines on each page.
Fabrication of flexible, electronically written display sheets are disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent indium-tin-oxide (ITO) conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be glass, but in practice have been formed of Mylar polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid crystal material. Electrical potential applied to opposing conductive areas operate on the liquid crystal material to expose display areas. The display uses nematic liquid crystal materials which ceases to present an image when de-energized.
U.S. Pat. No. 5,223,959 discloses a plurality of polymer dispersed liquid crystal material, each having a different dye material of red, green or blue dye material. Differing electrical signals to common electrodes operate on each of the materials to control the state of each type of dyed liquid crystal material. The patent requires the use of conventional nematic liquid crystals with a dye to absorb light. The droplets are chemically treated to be stable in either a clear or a light absorbing state. The invention also requires materials having different response times to electrical signals. The device must be continually driven so that the human eye perceives complementary colors. This arrangement has the disadvantage of requiring continuous, high speed electrical drive because the materials do not maintain their state. The material must be driven to achieve a neutral color density.
U.S. Pat. No. 5,437,811 discloses a light modulating cell having a polymerically dispersed chiral nematic liquid crystal. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal-conic state. The structure has the capacity of maintaining one of the given states in the absence of an electric field.
U.S. Pat. No. 5,847,798 discloses a continuous tone, neutral density display which can be driven between a reflective and light absorbing state. The display use a single cholesteric material to provide a constant reflectance across the visible light spectrum to provide a neutral color. The material is reflective in the non-visible spectrum and appears clear in the visible spectrum. The display can also be driven in to a stable light scattering state that is uniformly scattering in the visible spectrum. A neutral xe2x80x9cwhitexe2x80x9d state occurs, but the provides a very low reflective intensity.
Currently, privacy windows are created using the scattering properties of conventional nematic liquid crystals. Such materials require continuous electrical drive to remain transparent.
It is an object of the present invention to provide a neutral density sheet with memory properties.
It is another object of the present invention to provide a neutral density sheet which permits continuous tone, neutral density images that are stable in a zero-field condition.
A still further object is to provide a sheet usable as a display that can be electronically written repeatedly.
These objects are achieved in a multi-layer, neutral-density sheet with memory properties, comprising
(a) a transparent substrate;
(b) a transparent, electrically conductive layer formed over the transparent substrate; and
(c) a plurality of light modulating layers formed over the transparent electrically conductive layer, each light modulating layer being formed of polymer with a dispersion of cholesteric liquid crystal material having memory properties and selected so that in combination with cholesteric liquid crystal in other layers are controllable between a first, light reflecting neutral-density state and the second transparent state.
The present invention provides a neutral density privacy sheet that is stable in either a light blocking or light transmitting field in a zero state condition. The reflective characteristic of cholesteric materials provided in multiple layers generates a light reflection across the visible spectrum.
Sheets made in accordance with the present invention can be used to provide a re-writable image display sheet. The present invention uses a plurality of layers of cholesteric liquid crystal materials that are effective in at least two states, a reflective state and a transmissive state. This invention permits the use of light modulating, electrically responsive sheets with improved reflective efficiency. The sheet can be formed using inexpensive, efficient photographic layer methods. A single large volume of sheet material can be coated and formed into various types of sheets and cards. Displays in the form of sheets in accordance with the present invention are inexpensive, simple and fabricated using low-cost processes.